Aqueous polyvinyl alcohol stabilized polymer emulsions comprising ethylene vinyl acetate polymers find widespread applications as adhesives in the areas of packaging and converting and for vinyl laminating. For these uses, the adhesives are prepared and employed in emulsion form and, on removal of the aqueous medium, cure or harden at room temperature to form a bond which is desirably characterized by high strength and resistance to heat, humidity and water.
In recent years, there has emerged an increasing need for higher solids emulsions which will provide faster setting times for use on high speed production equipment. High solids adhesive bases may also find use as replacements for conventional hot melt (100% solids) material which require elevated temperatures with consequent expenditures of considerable energy. In addition to the need for high solids emulsions for such applications, it is also essential that the emulsion remain sufficiently low in viscosity that they can be applied using conventional equipment. Typically, desirable coating viscosities are in the range of 800-5000 cps at 25.degree. C.
Various methods have been described for the preparation of vinyl acetate based emulsions at solids levels of about 60%-65%. These methods are described, for example, in U.S. Pat. Nos. 4,921,898, 5,070,134, 5,629,370 and EP 389,893 and generally rely upon the use of modified polyvinyl alcohols and/or specific surfactant systems. The methods described in these patents have not, however, been particularly useful in the preparation of emulsions at solids levels greater than about 65% by weight.
It is an object of the present invention to provide a process for the production of vinyl acetate based homo and copolymer emulsions at high solids levels, i.e., in excess of 65% and preferably greater than 70% solids while maintaining relatively low coating viscosities in the range of 1000 to 8000 cps, preferably 2000 to 5000, at 25.degree. C.